The present invention relates to a process for removing simultaneously ethylene and vinyl chloride from a gas containing them.
In many processes for chlorination or oxychlorination of ethylene, the conversion of ethylene is not substantially complete. In particular, an effluent from a conventionally used oxychlorination reactor usually contains 0.1% to 10% by volume of the unreacted ethylene together with hydrogen chloride gas, oxygen and an inert gas in addition to the reaction product even after separation of the condensate by cooling for liquefaction. In addition to the unreacted ethylene, 50 to 500 p.p.m. of vinyl chloride is usually further contained in the effluent gas. From view point of prevention of environmental pollution, it has been lately desired to make the concentrations of ethylene and vinyl chloride in such waste gases as low as possible.
With respect to ethylene, there has hitherto been proposed an ethylene recovery system for recovering the large portion of the residual ethylene, as disclosed in Japanese Patent Examined Publication (Tokkyo Kokoku) No. 43921/1974 and Japanese Patent Unexamined Publication (Tokkyo Kokai) Nos. 57105/1977 and 59105/1977.
Japanese Patent Examined Publication No. 43921/1974 discloses a process in which an ethylene purification reactor is installed at the outlet of an oxychlorination system for ethylene and the unreacted ethylene in the oxychlorination effluent gas is reacted with chlorine in the presence of activated alumina as a catalyst to produce 1,2-dichloroethane. However, in Japanese Patent Unexamined Publication No. 59105/1977, it is pointed out that according to the above-mentioned process at least several thousands p.p.m. of ethylene still remains, and as a process for remarkably decreasing the residual proportion of ethylene, there is proposed a process in which a reactor charged with a mixed catalyst of metal iron and activated alumina impregnated with ferric chloride is further installed to decrease the ethylene concentration to less than 50 p.p.m. However, in that case, a relatively long contact time is required in catalytic reaction, and accordingly the reactor must be made large-sized and a heavy equipment investment is required. Also, metal iron may rapidly react with chlorine, and the resulting ferric chloride may transpire from the reactor or cause troubles such as choking of the reactor. Also, the removal of vinyl chloride is not intended and no description as to vinyl chloride is seen in Japanese Patent Unexamined Publication No. 59105/1977. In fact, according to the study of the present inventors, this process cannot sufficiently remove vinyl chloride, and it was also observed that under a certain reaction condition vinyl chloride is produced by catalytic decomposition of 1,2-dichloroethane contained in the gas stream, resulting in the increase of the content of vinyl chloride in contrast with the removal.
Some process for removing vinyl chloride included in a gas are proposed. One of them is an adsorption process using active carbon, but the process has problems that it is hard to decrease the concentration of vinyl chloride in a gas to less than about 20 p.p.m. and that the escape of vinyl chloride to the atmosphere cannot be prevented in a steam reproducing cycle and upon disposal of active carbon. Particularly, in case that a gas contains a chlorinated hydrocarbon other than vinyl chloride like a waste gas from a vinyl chloride manufacturing process, the adsorption process is economically disadvantageous, since the adsorption capacity for vinyl chloride is remarkably lowered.
As another process for removing vinyl chloride from an inert gas stream, there is known a process in which the gas stream is subjected to combustion to decompose vinyl chloride to carbon dioxide, water and hydrogen chloride and the exhausted gas is then washed with a caustic alkali to remove hydrogen chloride. However, not only this process is economically disadvantageous due to large apparatus and fuel costs, but also the combustion waste gas pollutes the air.
There is proposed the improvement of the process of Japanese Patent Examined Publication No. 43921/1974, in which the conversion of ethylene can be increased and simultaneously the selectivity to 1,2-dichloroethane can be remarkably increased by the use of activated alumina supporting cupric chloride and/or ferric chloride as a catalyst, as disclosed in Japanese Patent Unexamined Publication No. 57105/1977 in which one of the present inventors is concerned. It is known that ferric chloride catalyzes the chlorination mechanism. However, even if ferric chloride is supported on porous carriers having a particle size of about 5 to about 6 mm. which are usually known as carriers for catalysts used in fixed-bed reactions, it slightly accelerates the chlorine-addition reaction of vinyl chloride. Therefore, it has been difficult to decrease vinyl chloride to 20 p.p.m. or less concentration and moreover to maintain the catalytic activity in the initial stage of the use over a long time.